1. Field of the Invention
The present invention relates to a superconductive magneto-resistive device, and more particularly, to a superconductive magneto-resistive device comprising laminated superconductive ceramic films.
2. Description of Related Art
Conventionally, a magnetic sensor utilizing a magneto-resistive device made of a semiconductor or a magnetic material has been widely used for measuring a magnetic field. In this sort of magnetic sensor, a device utilizing a form effect such as a semiconductor of InSb, InAs etc. having a high electron mobility and a device utilizing an orientation effect such as a ferromagnetic material of Fe-Ni (permalloy), Co-Ni etc. have been put to practical use. In the conventional magnetic sensor utilizing the magneto-resistive device, the resistance R of the magneto-resistive device increases along a parabolic curve as shown in FIG. 1 when a magnetic field intensity H to be measured is changed in such a state that a constant current I is supplied thereto. Therefore, when the magnetic field intensity H is relatively low, the precision upon measuring a relatively low magnetic field intensity H is lowered since the variation ratio of the resistance to the magnetic field intensity is relatively small.
Further, the SQUID (Superconductive Quantum Interference Device) has been used as a magnetic sensor. The SQUID has an extremely high sensitivity to measure a low magnetic field of about 10.sup.-10 gauss and utilizes the Josephson device wherein an extremely thin insulation film is arranged between superconductive films. However, there are such disadvantages that it is difficult to make the Josephson device since it is made by using a difficult technique and it is necessary to amplify a voltage generated therein since the level of the output voltage is relatively low.
Furthermore, a magnetic sensor utilizing a magneto-resistive device of superconductive ceramic films comprised of grain boundaries has been proposed by the present applicants. In this sort of magnetic sensor, there is used either a superconductive ceramic film comprised of grain boundaries which either an insulation film where grain boundaries of a superconductor are extremely close to each other or a normal conductive film is arranged, or a superconductive ceramic film comprised of grain boundaries which are supposed to form point type weak couplings. Even though a relatively low magnetic field is applied to the ceramic superconductor, some of the superconductive state of the above grain boundaries are broken, resulting in that the ceramic superconductor becomes the normal conductive phase from the superconductive phase, and the ceramic superconductor has an electric resistance.
FIG. 2 shows a magneto-resistive characteristic of the above magnetic sensor utilizing the above ceramic superconductor having weak couplings between the grain boundaries, the ceramic superconductor being made by a conventional method for sintering powders of the materials thereof.
As shown in FIG. 2, when a relatively low magnetic field is applied to the ceramic superconductor, some of the above weak couplings between the grain boundaries at the superconductive state are broken, and the resistance thereof increases steeply according to the increase of the magnetic field intensity. This implies that the magnetic sensor utilizing the ceramic superconductor has a high sensitivity even though a relatively low magnetic field is applied thereto.